This invention relates generally to radio frequency energy systems and more particularly to a system for selectively transmitting or receiving radio frequency energy in one of a plurality of directions.
In many radio frequency systems, it is desirable to transmit or receive signals in any one of a plurality of directions. For the sake of simplicity, only the receive case is discussed here, but all statements could equally well cover the transmit case. Often, the radio frequency system is in a fixed location and the desired signal at any given time could come from any angle within a range of angles relative to the antenna.
One known way to receive a signal selectively from any of a plurality of angles is by electronically "steering" an array antenna. The angle to which the antenna is "steered" is determined by appropriately combining the signal as received at each array element. Before combining the portion of the signal received at each element, an appropriate phase shift is introduced into each portion of the signal.
One way of providing the appropriate phase shift is by employing an electromagnetic lens. Each antenna array element is connected to an array port along the front wall of the lens. Beam ports are disposed along the back wall of the lens. When the antenna is used to receive signals, the receiver is connected to a selected beam port. As is known, the antenna array forms a high gain receive beam pointed in the selected direction.
A signal impinging on the antenna array is coupled through each antenna element to each array port. From each array port, a portion of the received signal propagates along a path through the lens to the beam port. At the beam port, then, the portions of the signal in the various paths are combined.
The portions of the signal combined at the beam port are shifted in phase relative to each other. This occurs because the length of the paths from the source to the beam port can be different. Each length difference is proportional to a phase difference, with the constant of proportionality being the wavelength of the signal.
As is known, the strength of the combined signal at the beam port depends on the angle from which the signal impinges on the antenna array. The walls of the lens along which the array ports and beam ports are disposed are curved. The radius of curvature of the back wall is selected such that the back wall is along the "focal arc" of the lens. Portions of a signal impinging on the antenna from any given angle travel along the various paths in the lens such that the portions of the signal in the various paths arrive all with essentially the same phase at one particular point along the focal arc. Since the portions of the signal are combined with the same phase, they will produce a maximum signal level at this particular point.
A beam port located at a point along the focal arc is deemed to receive signals from the angle that results in the maximum signal level. The beam port is thus said to correspond to an angle.
However, the signal received at a beam port represents not just the signals received from the corresponding angle, but also signals received from closely related angles. However, the signals received from closely related directions are attenuated more than signals from the specific angle. The further from the specific angle the signals come from, the greater is the attenuation. For this reason, the antenna array is said to form a receive beam. The angle from which the maximum signal level is received is said to be the "beam center". The beam has a "width" which covers all angles from which signals are received with less than 3 dB more attenuation than at the beam center. A signal falling within the beam will be attenuated so little that it is deemed to be received by the system.
To receive signals from any angle in a range of angles, enough beam ports are located along the focal arc such that a plurality of beams is formed. Every angle in the range is included in at least one of the beams. To selectively receive a signal from a particular direction, a receiver is connected to the beam port corresponding to a beam in that direction.
One drawback to this approach is that connecting one receiver to each beam port can be very expensive. Even if one receiver is used and switched between the various beam ports, the switching apparatus to connect a receiver to any one of a plurality of beam ports can be very complicated and expensive. In general, the switching apparatus is more complicated and expensive when more beam ports need to be connected to the receiver. It would, therefore, be desirable to minimize the number of beam ports.
The number of beam ports needed in any system will depend on two factors: the range of angles in which the beam must be steered and the maximum beam width that can be used in the system. For example, in some systems, it may be necessary to distinguish between signals received in directions separated by as little as 10.degree.. In that case, each beam could have a width of no more than 10.degree.. The beam width of the beam corresponding to each beam port is determined by the length of the antenna array. It would seem that the number of beam ports would be the range of angles divided by the maximum allowable beam width. However, this is not the case. The width of each beam is not the same. Beams in directions near the broadside of the antenna are narrower than beams directed off broadside. If the length of the antenna is selected to provide the required beam width for the widest beam, the beams near the broadside of the antenna will be much narrower than required. Consequently, more beams, and more beam ports, are required in directions near broadside of the antenna.
In phased array antennas, phase shifters can be appropriately controlled to ensure that the beam width is the same regardless of the direction in which the beam is steered. However, a phased array antenna is not suitable for use in all systems. For example, where more than one receive beam must be formed simultaneously, a phased array system could be more complicated and expensive than a system using a beam forming lens.